Add option to disable window reordering by removing the dock window title bar

Add option to remove the dock widget title bar. The main benefit is to get back some screen area that is used by the title bar.

The change is based on discussion in http://forum.openscad.org/Performance-regression-2014-10-02-td9989.html.

With the undock option this effectively enables 3 cases:

- No reorder possible due to missing title bar (reorder = false, undock = false)
- Reorder but no undocking, title bar is visible (reorder = true, undock = false)
- Reorder and undocking of windows is possible (reorder = true, undock = true)